Surrounded by talkative people, it can be really hard to notice when someone is addressing me.
Having the ability to highlight predefined words (like 'Diz,' for example!) would make it a lot easier for people to pick a conversation or compliment or remark out of a busy history screen.
When I say 'people,' I mean me, of course.
This'll particularly help when I already have a pile of IM windows open and might otherwise miss something in the main chat window.